Here's a longer explanation, and I'd appreciate if you'd create a wiki topic we could link to in future, especially as this problem isnt related to Grid but to code by blizzard (SecureRaidGroupHeader, contributed by Esamynn, Cide, and Iriel) that we have to use:
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Well, the answer is simple. You cannot reposition these frames when in combat. So lets assume you're raiding AQ20 and have 2 priests, 2 shamans and one druid. You'd put the three frames underneath each other. Now what happenes when another priest joins? He'll overlap the next group, in other words you'll have two unit frames on top of each other. Even if you'd now say "hey, I can live with that - if I'm raiding, noone joins/leaves in combat" - well, you can't really delay the change of the units until the second you're out of combat. And, what would happen if you were in AV, where any other second someone leaves/joines? A big mess.
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Possible changes Blizzard could make:
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Have you noticed that every day, 1-3 people come here begging for the same 2-3 custom layouts? And the response is "Blizzard Secure Raid Headers don't allow it."
Sure, and without the Blizzard Secure Raid Headers, you can't reposition in combat.
So what? How about a best-effort?
Can't you just write 40 buttons. Not even using the raid headers. Those buttons can look any way you want them to, and each possible click on them can do anything you want it to. Target, cast, whatever. As long as you set it up before combat.
So every single layout these people ask for would work. It would just have some weird annoyances if people join or leave while you're in combat, since Blizzard's frames update during combat, and yours can't. So what? Who cares? Your example of frames overlapping if a warrior leaves and a priest joins would be annoying, yes. So why would you implement it that way? Why not have 40 buttons, set up to display the way we want them to display, and sorted the way we want them sorted pre-combat. Any one of the ways people want them -- sorted by class forced into a 5x8 (so allow overflow), or rows by class sorted by name, or anything. Then if your priest leaves and is replaced by a warrior during combat, then the warrior is stuck in the priest group on your interface in the rwong alphabetical order until the combat ends. That's really not that bad of a situation to be in. In PvP, it's not like you're expecting a casual clean interface -- I make anyone who's out of range in PvP go 5% alpha anyway, so if the warrior in the priest group just joined? I won't even see that it's not the priest any more until long after I drop out of combat for a second.
And obviously, when I'm rocking Naxx? People don't join and leave during combat. Unless something is going very, very wrong. And if something is that wrong, hey! who cares! Once the wipe's over, my interface'll update. :)
I mean, I can accept the argument "With the way Grid is set up right now, that would be a lot of work and really annoying to maintain." But I have yet to see an argument that actually explains to me why it's not possible to have a 100% customizable layout option. And for people who really don't want to ever see a warrior in the middle of the priest group under any circumstances, there's still the sorting options that are available right now.
PS theoretically, since I haven't actually *written* a grid-like mod that does what I was saying is possible, I could be wrong, and if so, I'd like to know why... what part of what I described isn't possible? :) Thanks!
PPS I'm a fan of the current sorting systems, myself, and would never use the ideas I described, I only ask this 'cause it seems like it should be possible given everything I know about the secure system, and despite tons of requests for it, there seems to be this outright denial of it even being possible, so I'm interested in knowing why it's impossible, or why you say it is. :)